The use of CT (computer tomography) imaging methods in medicine is now widespread. It is also commonplace to attach frames or mechanical devices to the patient during CT scanning. A common application is in brain surgery where a head ring is attached to the patient""s skull for the purpose of providing a reference platform at the time of CT image scanning. Typically, the head ring is fastened directly to the patient"" skull by head posts and skull fixation means, such as sharpened, pointed screws on the head posts that anchor directly to the skull. This frame may then be used as a rigid platform onto which may be attached a localizer structure. When a patient is scanned with the localizer structure so rigidly attached to his skull, index marks from the localizer will appear on the scan slices and provide means for mathematically determining the coordinates of every image point seen in the CT slice relative to the head frame. This technique was described in detail in U.S. Pat. No. 4,608,977 by R. A. Brown, patent issued Sep. 2, 1986. This methodology has been commercialized successfully by Radionics, Inc. in the form of the BRW Brown-Roberts-Wells Stereotactic Instrument. The method has been successfully used for X-ray-based, CT, MRI, and P.E.T. scanning computer tomographic scanning.
A difficulty with this technique is that it requires placement of the head frame to the patient""s head prior to CT scanning, and, in the surgical phase, the head ring must remain on the patient""s head between the time of scanning and the time of surgery. This is satisfactory for surgery such as brain biopsy when the stereotactic biopsy surgery will be done within hours of the CT imaging. Typically it will involve putting the head frame on, attaching the localizer system to the head frame, performing the CT image scan to determine the index fiducial points of the localizer structure on the CT image planes, doing a calculation to relate mathematically the image data via the fiducial points to the coordinate frame of the head ring, and attaching a stereotactic arc system or guidance means to the head frame so as to achieve the calculated target with an instrument or probe, and thus, to do the biopsy. A similar methodology is practiced for so-called radiosurgery in which a similar procedure is carried out, except that the surgical probe is actually a beam of photons, and the guidance means is an accelerator which provides the source of photons. These photons are beamed to the target for treatment or destruction of the target volume by attaching the head ring directly to the accelerator and moving the head ring in space so that the target volume stands at the confluence of the photon beam paths.
It would be of significant advantage to be able to carry out the CT image data without having to place the head frame on the patient""s head, rather than to have the head frame remain on the patient""s head between the time of scanning and the time of surgery or radiosurgery. For instance, it may be wished to analyze the CT scanning data over an extended period so as to plan for a complex surgery or radiosurgery. It may also be desired to do repeat surgery or radiosurgery many times after image scanning, and extended over periods of days or weeks, making it inconvenient or impractical to leave the head ring attached to the patient""s head for that period. Thus, it would be desirable to perform the CT image scanning at one time, and to be able to relate the image scanning data at any time in the future to a head frame or fixation means which may be applied to the patient at that later time of surgery or scanning. For example, a surgical head holder may be applied to the patient""s head at the time of surgery after the patient has been anesthetized. It would be desirable to be able to relate the position of that head frame to the volumetric image data that has been acquired from the CT imaging procedure at a previous or at a subsequent time. Furthermore, it may be desirable not to attach the head frame to the patient at any time, but rather use so-called frameless stereotactic means, such as an operating arm, or other device, to access quantitatively volumes of targets in the patient""s head based on the CT image data which has been stored and manipulated in a computer.
Thus, one specific implementation of the present invention relates to a method of achieving the above-state goal of being able to perform CT imaging without a head frame, accumulate that data in a computer, subsequently have the patient in the operating setting or radiosurgery or radiotherapy setting and, at that time, relate the CT image data to a coordinate reference frame in the operating theater or radiation suite, such as, for example, a head frame put on at the time of surgery. No head frame would need to be attached during the CT imaging or at the time of surgery necessarily, however, in one embodiment of the present system, a head holder may be applied at the time of surgery or radiation surgery, and the coordinate system related to that head holder is mathematically related to the coordinate system of the CT scanning machine by means of X-ray or optical imaging carried out at the time of surgery. In one embodiment, index markers may be placed on the patient""s skin or attached to the patient""s skull as an example of a means of defining a specific coordinate system related to the CT imaging itself. At the time of surgery, if a head frame is attached, X-ray images of the patient""s head may be taken, and the index markers that have been attached to the patient""s head may be visualized relative to index or fiducial points attached to the head frame. By analyzing the relative position of the patient-attached index points to the head frame-attached index points, a transformation can be made to relate the respective CT image coordinate frame to the coordinate frame of the head frame, the latter being the surgical or radiosurgical coordinate frame. Thus, in one embodiment, the present invention relates to a transformation from CT image coordinate frame to surgical coordinate frame at the time, or approximately the time, of surgery. The surgeon is thereby freed from the need to place a head frame onto the patient at the time of imaging, and subsequent surgical episodes can be done repeatedly with very precise reproducible means of mapping the CT image field onto the surgical field.
An alternate example of the present invention is to place the index marks on the patient""s skin or to the patient""s bone anatomy, such as the skull, using a frame or guidance means so as to put the markers in a specific, relative orientation. One example would be to put them on the principal axes of a Cartesian coordinate system relative to the skull. This could easily be done by an appropriately designed implantation guide device. The index marks then can be visualized during surgery at a subsequent time when another head frame is put on, or no frame is put on at all. The index marks may be visualized by external apparatus, such a linear accelerator collimator system or a fluoroscopic X-ray machine or some other surgical device. The position of the external apparatus may be thusly determined relative to the body. CT scanning can be done subsequently, and the CT data can be related to that Cartesian coordinate system. This is yet another embodiment of placing index markers in the head or the body so as to relate CT, X-ray, and other image modalities one to the other.